arm64, riscv: Set VM_KMEM_SIZE_SCALE to 1

This setting limits the amount of memory that can be allocated to UMA.
On systems with a direct map and ample KVA, however, there is no reason
for VM_KMEM_SIZE_SCALE to be larger than 1. This appears to have been
inherited from the 32-bit ARM platform definitions.

Also remove VM_KMEM_SIZE_MIN, which is not needed when
VM_KMEM_SIZE_SCALE is defined to be 1.[*]

Reviewed by: alc, kp, kib
Reported by: alc [*]
Submitted by: Klara, Inc.
Sponsored by: Ampere Computing
Differential Revision: https://reviews.freebsd.org/D28225